1. Field of the Invention
The present invention relates to a receiving apparatus used for a random access channel of a mobile communication system, and more particular, to a random access channel receiver in a base station of a code division multiple access (CDMA) mobile communication system.
2. Description of the Related Art
A random access channel is provided between a base station and a mobile terminal for conveying various kind of control request signals from the mobile terminal at random basis. A call request signal at the time of call origination by the mobile terminal, a hand-off request signal during communication, and a location registration request signal when detecting new radio area during movement are some examples of control request signals transmitted by the mobile terminal. A random access channel is commonly used by a plurality of mobile terminals, and a random access channel receiver in the base station is used for receiving control request signals on the random access channel transmitted from a plurality of mobile terminals which request the foregoing control operations.
FIG. 1 is a block diagram showing a random access channel receiver used in the base station of the CDMA mobile communication system. Control request signals transmitted by mobile terminals 1, 2 are received by a base station 3. Each signal is passed through a RF (radio frequency) amplifier 31, a frequency converter 32, and input to a signal decoding unit 33 (,conversion of an analogue signal to a digital signal is performed before inputting the signal to the signal decoding unit 33). In the signal decoding unit, the received signal is demodulated by a specific spread code and output to an information demodulator 34. Information of the signal is obtained by demodulating the signal by the information demodulator 34.
FIG. 5 is a block diagram showing the signal decoding unit of the conventional random access channel receiver.
As shown in FIG. 6, the signal decoding unit consists of a plurality of signal decoding parts 601-60N provided for each off-set timing of received signal from the mobile terminal. off-set timings are provided because the random access channel is commonly used by a plurality of mobile channels which means the same spread code is used by all mobile terminals, and if no countermeasures is considered, the signal on the random access channel is interfered each other. Therefore, an intentional delay (or shift) of a few symbols is applied as an off-set timing, and 4-16 off-set timings, depending on the mobile communication system, are provided for the random access channel. When the mobile terminal transmits the control request signal on the random access channel, the mobile terminal selects one of the off-set timings. Then, the base station can discriminate each signal by establishing synchronization.
Backed to FIG. 6, the signal decoding unit of the conventional random access channel receiver is provided with a plurality of signal decoding parts, each establishes synchronization with each off-set timing independently. The signal decoding part 601 comprises a received data accumulating memory 61 for receiving signals from the frequency converter; a synchronism establishing circuit 62 for establishing synchronization with predetermined off-set timing and delivering a synchronizing signal; a correlator 63 for demodulating (or de-spreading) the received signal in the received data accumulating memory 61 by the specific spread code for the random access channel on the basis of the synchronizing signal from the synchronism establishing circuit 62; and a decoding circuit 64 for decoding the output of the correlator 63 and outputting decoded signals to the information demodulator.
As described above, the conventional random access channel receiver should be provided with a plurality of signal decoding parts for each of off-set timings. The reason is that, the time range for establishing synchronization is set to be very short relative to a signal frame for the purpose of preserving a real time operation feature, and it is accordingly difficult for a single signal decoding part to cope with a plurality of off-set timings varying on a frame-by-frame basis.
An object of the present invention is to provide, in view of the difficulties of the conventional random access channel receiver, a random access channel receiver with a single signal decoding part for performing burst reception of control request signals having different off-set timing each other.
A receiving apparatus for a random access channel in a base station of a CDMA mobile communication system, which receives a plurality of signal frames with a respective off-set timing transmitted by a plurality of mobile terminals, the receiving apparatus comprises a synchronizing circuit, a synchronism determining and establishing circuit and a correlator.
The synchronizing circuit calculates and outputs each correlation value at each off-set timing.
The synchronism determining and establishing circuit determines and outputs a synchronizing signal indicating an off-set timing, whose correlation value exceeds a predetermined threshold value, and is higher than any other correlation values corresponding to other off-set timings provided for this mobile communication system.
The correlator demodulates each signal frame being arrived by a predetermined spreading code for the random access channel in accordance with a synchronizing timing instructed by the synchronizing signal output from the synchronism determining and establishing circuit.
In more detail, the synchronizing circuit receives a plurality of signal frames being arrived, calculates a correlation value between each signal frame and a predetermined synchronization word at each off-set timing, and outputs the calculated correlation value for each off-set timing; and a synchronism determining and establishing circuit determines an off-set timing, corresponding correlation value exceeds a predetermined threshold value, as a synchronizing signal, and outputs the synchronizing signal, and when the correlation value of the succeeding off-set timing indicates a higher correlation value, it resets the synchronizing signal to new synchronizing signal. Therefore, the correlator, which performs demodulation operation for each signal frame being arrived by a predetermined spreading code for the random access channel with the synchronizing signal, abandons the demodulation operation for a signal frame being demodulated and initializes the demodulation operation by new synchronizing signal when the synchronism determining and establishing circuit has reset the synchronizing signal.
This makes possible to receive a plurality of signal frames each having different off-set timing with a single decoding part, and enables the receiving apparatus to be reduced in scale.